The present invention relates to a screw press. A screw press is a press machine which has a servo motor as the driving source and raises and lowers a ram (a slide) by a screw mechanism.
U.S. Pat. No. 2,533,486 is an example of the prior art. In the patent publication for this prior art example, there is described a construction in which an electric motor is the drive source and a ram is raised and lowered via a screw mechanism. The position and speed and the like of the ram are controlled using pulses from an encoder which is provided on the motor.
Referring to FIG. 5, this prior art is shown. A screw shaft 7a is connected to the output shaft of a motor (servo motor) 2. An end of screw shaft 7a is screwed into a nut 8a of a ram 8. When servo motor 2 rotates in the direction of arrow B, ram 8 is raised and lowered in the direction of arrow A due to the action of a screw mechanism 7. In other words, when there is positive rotation and reverse rotation of servo motor 2, ram 8 has a reciprocating motion due to the action of the screw mechanism. In this situation, because the screw shaft is provided perpendicularly, ram 8 has a rise and fall motion.
An upper mold (not shown) is affixed to ram 8. Opposite this, a lower mold is affixed to a bolster. Material supplied between these upper and lower molds is pressed.
As described previously, an encoder 3 is provided on servo motor 2. There is feed back of the pulse signals of encoder 3 to a position control circuit and a speed control circuit, and the position and speed of ram 8 are controlled.
Because the rotation amount of servo motor 2 is detected by encoder 3, the amount of raising or lowering of ram 8, or, in other words, the position of ram 8, can be controlled by the pulse number from encoder 3. Furthermore, the speed of ram 8 can be controlled by the pulse number per unit time of encoder 3.
In the same figure, processing of data, such as inputting, establishing, changing, and the like of data such as the position, speed, and the like of the ram, is conducted by the information processing control circuit. By matching the pulse of encoder 3 with a position and speed of ram 8 which have been established beforehand, the position and speed of ram 8 can be controlled.
The above control system is called a semi-closed loop system. All of the control of the ram is conducted by the pulses of the encoder. There are no means for feedback of the actual movement conditions of the ram. As a result, in this prior art example, in order to have a high precision position control of the ram, there is direct coupling of the motor, which is the driving source, and the screw mechanism. There are no devices interposed between them. Such devices can obstruct the precision of the ram position because of deformation or loosening.
Stated differently, in the semi-closed loop system of the prior art example, because there is no construction for feedback of the actual positional information of the ram, if any machinery is placed between the motor and the screw mechanism, the rotation amount of the motor does not directly translate into the position of the ram, and the desired positional precision for the ram is not achieved.
The object of the present invention is to provide a screw press which can ensure the positional precision of the ram when the rotation amount of the above motor does not directly translate into the position of the ram because there is other machinery, such as a decelerator and the like, present between the motor (servo motor) and the screw mechanism.
The present invention has a construction in which a position detection sensor, which detects the position of the ram, is provided, and the position of the ram is controlled by the output from this sensor. The invention is directed to a screw press, in which a ram is driven by a servo motor via a screw mechanism, wherein a position detection sensor, which detects the position of a ram, is interposed between either a frame or a bolster and the ram; output from the position detection sensor is used in positional control of the ram; the position in the vertical direction of the positional sensor is near the lower surface of the ram. In addition, the invention provides a screw press, wherein: with respect to the value of the bottom dead center which has been established beforehand, an in-position value in which the ram is considered to have reached the bottom dead center is provided, and the in-position value can be freely adjusted. In addition, the screw press is one wherein: in the vicinity of a plurality of points specifying ram positions which have been established beforehand, the variability for the control can be adjusted for each of the plurality of points by a timer. Among the plurality of points established beforehand, for each point except the bottom dead center, there are in-position values, with respect to positional data which have been established beforehand, in which the ram is considered to have reached each of the plurality of points, and the in-position values can be adjusted freely.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.